1. Field of the Invention
This invention relates to transistors and, more particularly, to amorphous thin film transistors.
2. Description of the Prior Art
An article by A. B. Fowler entitled "Thin Film Technology Part III -Active Thin Film Devices," IEEE Spectrum, June 1964, pp. 102-106, 111 describes thin film transistors (TFT) composed of polycrystalline CdS, CdSe, GeS and PbS, and the hypothetical crystalline semiconductor, metal base transistor is discussed.
U.S. Pat. No. 3,748,501 of Fritzsche et al entitled "Multi-Terminal Amorphous Electronic Control Device," describes a three-terminal threshold (transistor-like) control device which involves the variation of bulk operation of a bistable threshold device. It makes no suggestion of a continuously variable transistor of the N-P-N type, but relies upon hot electrons modulating the bulk conductance of certain other types of amorphous material. An emitter layer 24 of sputtered aluminum metal is deposited upon an insulating substrate. Then there is a layer of sputtered thin film material 28 composed of an insulating material such as Al.sub.2 O.sub.3, about 75-200A thick, transparent to electrons. A thin film sputtered metallic base electrode 16 is about 75-200A thick and composed of Al or Mo. A sputtered amorphous insulative film 12 about 7500-20,000A thick, has a conductivity threshold characteristic. The materials could be (atomic) As (Te:Ge) (75:(90:10)), TeAsSiGe (40:35:18:6.75) with traces of In 0.25; TeAsGeS (28:34.5:15.5:22); among others. See Col. 2 of the patent. An ohmic contact of a metal such as Al or Mo is deposited upon the amorphous film. While a thin film metal base three-terminal threshold transistor-like switching control device with the variation in the bulk state operation of a bistable threshold device of U.S. Pat. No. 3,748,501, variable state, two-barrier type has been described, amorphous semiconductive film transistors have not been anticipated. Neither has the combination of reversed back-to-back Schottky devices provided here. The Fritzsche et al patent is directed to bistable amorphous devices which do not provide the "infinite" range of outputs as a function of base-electrode potential which can be achieved by such a barrier device as is described here.
Sze et al, "Hot-Electron Transport in Semiconductor-Metal Semiconductor Structures," Journal of Applied Physics, Vol. 37, No. 7, June 1966, p. 2690 teaches a monocrystalline structure with a thin film metallic base deposited upon a monocrystalline substrate of Si in mechanical contact with a crystalline silicon rod emitter. Such crystalline structures are not adapted for application because of the relatively enormous size of the silicon rod and the difficulty of achieving contact without destroying the thin metal film.
U.S. Pat. No. 3,872,492 of Robbins for a "Radiation Hardened Field Effect Transistor" describes use of amorphous silicon as a dielectric material but is not used as an active semiconductive material as in the present invention.
U.S. Pat. No. 4,015,282 of Shaw entitled "Solid State Amplifier Device and Circuit Therefor" describes a semiconductor sandwich with an amorphous layer 6 in the center. Shaw's device is based upon the bistable operation of an amorphous semiconductor and does not provide a continuously variable device as taught here.
Other U.S. Pats. found were Nos. 3,470,426; 3,877,049; 3,886,577; 3,886,578 (thin film deposition of a device) and 3,987,311 (amorphous semiconductors in thin film logic).
Wronski et al in "Schottky-barrier Characteristics of Metal-Amorphous Silicon Diodes," Applied Physics Letters, Vol. 29, No. 9, 1 Nov. 1976, pp. 602-605 describes how an amorphous silicon Schottky diode operates. This reference uses glow discharge deposited amorphous silicon and a Schottky barrier with Pd or Pt metal. No suggestion is made that the Wronski et al teachings can be applied to three-terminal devices.
The Sze et al article does not suggest that a collector (or an emitter) could be vacuum deposited upon the metallic base since it would seem that epitaxial deposition of a monocrystalline silicon film would not be possible upon a metallic base in view of the prior art (Lindmayer, "The Metal-Gate Transistor, " Proceedings IEEE, Vol. 52, 1964, p. 1751). Nothing in the prior art suggests the advantage of substituting amorphous silicon for the monocrystalline silicon or that it would produce a workable device.
Use of more than one layer of an amorphous semiconductor such as hydrogenated amorphous silicon in a thin film transistor is novel. A field effect transistor using only a single layer of amorphous silicon is described in Neudeck et al, Solid State Electronics, Vol. 19, Aug. 1976, pp. 721-729. Use of a thin film semiconductor in a metal base transistor is also novel. No prior art cited teaches use of two layers of amorphous silicon. No combination of two references suggests the device of this invention.